In microlithography (projection-transfer or projection-exposure) of a chip pattern using a charged particle beam, the pattern is defined on a reticle that must be divided into a large number of exposure units that are individually exposed. The main reasons for dividing the pattern are that (1) it currently is impossible to fabricate a reticle defining an entire chip pattern that can be exposed in one shot, and (2) it currently is impossible to fabricate a charged-particle-beam (CPB) optical system capable of projection-exposing an entire reticle pattern in one shot while reducing aberrations (especially off-axis aberrations) to a suitably low level over the entire field.
The divided exposure units on a reticle for CPB microlithography usually are termed “subfields.” Each subfield defines a respective portion of the overall pattern, and each subfield is projection-transferred individually. Typically, the subfields are arranged on the reticle and exposed in sequential order. On the lithographic substrate, usually a semiconductor wafer coated with a suitable “resist,” the subfield images are formed and located in a manner that results in the individual images being “stitched” together in a contiguous manner that forms the entire pattern after all the subfields are exposed.
Even though subfield images can be projection-transferred with high accuracy and precision, projection-transfer of a highly intricate pattern using a charged particle beam usually requires that beam “blur” be reduced as much as possible so as to achieve the necessary high resolution of fine pattern elements in individual subfield images on the substrate. It also is necessary to achieve the lowest possible variation in blur in individual subfield images. I.e., blur variation (termed “Δblur”) within each exposure field and from one exposure field to the next should be as low as possible. Otherwise, excessive Δblur causes excessive variation in the linewidth of pattern elements within individual exposure fields and from one exposure field to the next. Minimizing Δblur also simplifies the implementation of corrective action to reduce blur.
In order to increase the fineness of line widths in patterns transferred by CPB microlithography and to obtain higher throughput, the sizes of individual subfields has been increasing recently. As subfield areas increase, however, Δblur within individual subfield images also increases and thus becomes more of a problem.
The Δblur within an individual exposure field is plotted in FIG. 7, which is a graph showing the relationship between the position of the image plane and blur at various locations A, B, C, D, E within the exposure field (i.e., within a subfield). The abscissa is the position of the image plane on the Z-axis (parallel to the optical axis of the CPB optical system), and the origin corresponds with the Gaussian image-plane position. The ordinate is blur, wherein minimal blur in this instance occurs at the origin. The five curves correspond to the five locations A-E, respectively, and are labeled similarly. Point A is located at the optical axis. The curve denoted “A” indicates the manner in which blur changes, at the point A, with corresponding changes in the Z-position of the image plane. Point E is located at the edge of the subfield. The curve denoted “E” indicates the manner in which blur changes, at the point E, with corresponding changes in the Z-position of the image plane. The curves denoted “B”, “C”, and “D” are respective plots of blur at corresponding points B, C, D, located between the points A and E, at increasing distance from the optical axis, respectively.
In other words, at the Gaussian-image plane where a wafer normally would be placed, blur is minimal at the optical axis (curve A), and maximal at the edge of the sub-field (curve E). In this figure, the line OF is Δblur exhibited in an exposure field of a substrate (wafer) situated at the Gaussian-image plane. Moving the wafer to a position (on the Z-axis) displaced from the Gaussian-image plane (e.g., any of curves B, C, D, or E) increases minimum blur but also allows Δblur to be decreased. For example, by placing the wafer at the position “G” in FIG. 7, minimum blur is the magnitude of blur corresponding to the point “H,” and maximum blur is the magnitude of blur corresponding to the point “I”. Thus, Δblur corresponds to the length HI, which is shorter than the length OF.
However, in these conventional methods involving shifting the substrate position away from the Gaussian-image plane before making individual subfield exposures, there are limits to the extent to which Δblur can be reduced. Specifically, as the size of the exposure field has continued to increase, it has become impossible to reduce Δblur to the necessary extent using these methods.
Another way, suggested in the prior art, for minimizing Δblur involves offsetting, in advance of exposure, linewidths of pattern elements defined on the reticle. The offset is defined on the reticle according to the position (distance and direction from the optical axis) within the subfield during reticle preparation. However, converting reticle-preparation and reticle-design data in this manner into actual exposure data places an enormous data-processing burden on the data-conversion system.